1. Field of the Invention
The present invention relates to a crystalline silicon semiconductor device and its method for fabrication, and particularly relates to a crystalline silicon semiconductor device having a polycrystalline silicon layer oriented entirely in an uniformed manner and a method for fabricating it or a crystalline silicon semiconductor device and its method for fabrication in which the polycrystalline silicon layers can be efficiently formed.
2. Description of the Related Art
A semiconductor device in which a polycrystalline silicon is grown on a substrate such as of glass or the like is known for a material of an electric cell preferable for a solar cell. Since this semiconductor device is not required for a large area and high quality of a silicon substrate, it allows for a large amount of cost down, however, in order to presently obtain a semiconductor device of good quality, a quartz plate of thermal resistance must be used as a substrate, therefore, it is difficult to secure a costly advantage because the quartz plate is expensive.
As a method for solving this problem, a method in which a thin film of an amorphous silicon formed on a substrate is melted and crystallized by laser annealing and a polycrystalline silicon layer is formed on it has been proposed. This method has been disclosed in K. Yamamoto et al., IEEE First World Conference on Photovoltaic Energy Conversion (1994, in Hawaii), pp. 1575-1578, and according to this, since the rise of a substrate temperature is suppressed, the description indicates that the use of a lower cost substrate is possible.
However, according to this method, since it takes a lot of time for forming a bedding crystal film and a polycrystalline silicon layer, especially the growth rate of a polycrystalline silicon layer is slow, thereby resulting in costing large expenses and at the same time, furthermore, there is a large amount of economic expenses caused by higher use loss ratio of silicon raw materials, that has to be a costly as a whole.
As another method of advantageously growing a polycrystalline silicon layer, a method of amorphous silicon being polycrystallized by making amorphous silicon contact with metallic catalyst and heating it has been proposed by R. C. Cammarata et al., J. Mater. Res., Vol. 5, No. 10 (1990) p. 2133-2138.
It is indicated, according to this method, that forming a film of polycrystalline silicon can be performed at low temperature and high rate. Especially crystallization at lower temperature can be achieved, for example, by introducing a trace quantity of Ni metal and heating it.
Then, according to this method, in the case where a thin film just like a TFT element in the order of 100 nm thickness is a subject, it is ascertained by L. K. Lam et al., Appl. Pys. Lett., Vol. 74, No. 13 (1999) pp. 1866-1868 that crystallization proceeds a few xcexcm in the inplane direction, therefore, a crystal of high quality which is oriented quite well in the inplane direction can be obtained. Moreover, as a method of applying this orientation growth, a method in which amorphous silicon is crystallized by a metallic catalyst being selectively arranged nearby the position of a TFT element and by performing heating process to it and high performance is contemplated by forming an element with a grain of the crystal has been also proposed in Japanese Patent Application Laid-Open Publication No. 6-244104.
However, according to conventional methods shown here, since any one of them has a limitation involving with an area being crystallized, it is difficult to apply these methods to the production of a semiconductor device for a solar cell.
In a semiconductor device for use in a solar cell, although the thickness of a silicon film is required around 1 xcexcm since a sufficient optical absorption is required within a film, when such a thick film is a subject, an area where crystallization can be performed by conventional methods is only in the order of 100 xcexcm2. Even if a metallic catalyst is formed on the entire surface of an amorphous silicon layer of an area suitable for a solar cell and heating process is performed to it, a silicon layer thus obtained represents only an arborescent growth which is branched and heterogeneous, it is impossible to obtain a good silicon layer which is crystallized in uniformity.
Accordingly, the first object of the present invention is to provide a crystalline silicon semiconductor device having a polycrystalline silicon layer which is oriented in a uniformed manner on the whole area suitable for a solar cell in a semiconductor device in which a polycrystalline silicon layer is grown by using a metallic catalyst, and a method for fabricating it.
Moreover, the second object of the present invention is to provide a crystalline silicon semiconductor device at an advantageous cost in which a polycrystalline silicon layer having a predetermined thickness can be efficiently formed on a cheap substrate and its method for fabrication.
In order to achieve the above-described first object, the present invention provides a crystalline silicon semiconductor device characterized in that it comprises a substrate and a polycrystalline silicon layer formed by amorphous silicon layer provided on the substrate and heat-treated in the presence of a metallic catalyst, the polycrystalline silicon layer is consisted of a polycrystalline silicon layer which is grown by heat-treating the amorphous silicon layer in the presence of the metallic catalyst dispersed in a dotted shape at lower portion or upper portion of the amorphous silicon layer.
Moreover, in order to achieve the above-described first object, in a method of a crystalline silicon semiconductor device forming a polycrystalline silicon layer of a predetermined thickness on a substrate, the present invention provides a method of a crystalline silicon semiconductor device characterized in that an amorphous silicon layer of a predetermined thickness is formed on a metallic catalyst dispersed in a dotted shape on the substrate and the amorphous silicon layer of the predetermined thickness is crystallized into a polycrystalline silicon layer by heat-treating the amorphous silicon layer of the predetermined thickness.
Furthermore, in order to achieve the above-described first object, in a method for fabrication of a crystalline silicon semiconductor forming a polycrystalline silicon layer of a predetermined thickness on a substrate, the present invention provides a method for fabrication of a crystalline silicon semiconductor device characterized in that a metallic catalyst is dispersed in a dotted shape on an amorphous silicon layer of the predetermined thickness formed on the substrate and the amorphous silicon layer of the predetermined thickness is crystallized into a polycrystalline silicon layer by heat-treating the amorphous silicon layer of the predetermined thickness.
The above-described amorphous silicon layer, in most cases, is consisted of an intrinsic (i type) silicon, and polycrystal layer which is grown by this is also consisted of a substantially intrinsic silicon. Moreover, on both surfaces of this polycrystalline silicon layer, amorphous silicon layers of n-type and p-type which are different electrically conductive types are commonly formed. It is desirable that a polycrystalline silicon layer is formed in a thickness of more than 0.6 xcexcm in order to secure the optical absorption characteristic.
In the above-described method for fabrication, as means for dispersing a metallic catalyst in a dotted shape on a substrate, a method of providing a concave portion on the surface of the substrate and making the metallic catalyst positioned in this concave portion is easy to be performed. Concretely, a method in which salt solution of the metallic catalyst is applied and dried on the surface of the substrate providing a concave portion thereby leaving the metallic catalyst in a thick film state within the concave portion remained is secured one. As a concave portion, it is preferable that its cross section is V-shaped one. Moreover, a method in which a convex portion is formed on the surface of the substrate and a metallic catalyst covers on the convex portion is also preferable, in most cases, transparent electrodes are provided in concave and convex shapes on the substrate, these concave and convex portions are consisted of concaves and convexes of these transparent electrodes.
It is possible that another film is formed on the substrate covered by a film of a metallic catalyst and the metallic catalyst is exposed from a pinhole by forming a pinhole on this film thereby contemplating dispersion of dotted metallic catalyst instead of forming concave and convex portions on the substrate. In this case, as a shape of a pinhole, it is preferable to be a non-circular shape such as elliptic, square or rectangular shape, in the case where a pinhole is formed in such a non-circular shape, orientation of a polycrystalline silicon which is growing will be enhanced. Provided that formation of a pinhole of circular shape is not denied. A formation of a pinhole to other film on a metallic catalyst is easily performed by selecting condition of formation of film or laser beam machining and the like.
As other method for dispersing a metallic catalyst in a dotted shape, a method in which a thin film of a metallic catalyst formed on a substrate is heat-treated thereby aggregating the film of the metallic catalyst and dispersed places in dotted shape being formed by aggregated portion can be also contemplated.
A metallic catalyst dispersed in a dotted shape is not limited to be formed on the substrate. The metallic catalyst maybe dispersed at the upper portion of amorphous silicon layer formed on the substrate. As a method for dispersing the metallic catalyst in a dotted shape at the upper portion of amorphous silicon layer, the aggregation of the metallic catalyst film by means of the above-described heat-treatment is preferable.
In a method for fabrication of the present invention, it is preferable that a metallic catalyst to be dispersed in a dotted shape at the lower portion or upper portion of an amorphous silicon layer, in order to grow a polycrystalline silicon layer sufficient at the lower limit, in order to secure the effect of scattering of the metallic catalyst at the upper limit, is provided so that the metallic catalyst occupies 0.1-50% of an area of the lower or upper portion of the amorphous silicon layer. It should be noted that dotted shape indicating a state of formation of the metal may be dotted literally as they are or may also be scattered in a plane-like. Briefly, these are determined by the relationship with the amorphous silicon layer, and there is no limitation for its largeness.
It is preferable that heating-treatment for growing a polycrystalline silicon layer from an amorphous silicon layer is performed in the atmosphere of nitrogen, vacuum, hydrogen, Ar or halogen and the like. Moreover, although heat-treatment is commonly performed at a certain temperature, for example, it is possible to be performed in the form of heating at the predetermined heat treatment temperature after a level of hydrogen in a film is set less than 1% or preferably, less than 0.3% and the like by heating in the order of 400xc2x0 C. in the atmosphere of hydrogen, in the case where the heating treatment is performed in a stepwise manner, the orientation of a polycrystalline silicon layer obtained can be made better.
As a component of a metallic catalyst, it is preferable to be selected from Ni, Fe, Co, Pt, Cu, Au or a chemical compound such as alloy including them and the like. As a component of a substrate, a transparent material such as glass, ceramic, sapphire, quartz or the like or a metal material such as SUS, Al, tungsten, metallic silicon or the like is used. It is possible that light scattering effect is given by forming fine concave and convex on the surface of a metal substrate such as SUS or the like thereby contemplating an increase of short circuit current.
In order to achieve the above-described second object, the present invention provides a crystalline silicon semiconductor device characterized in that it includes a polycrystalline silicon layer of one electrically conductive type having the predetermined orientation formed on a substrate, a substantially intrinsic polycrystalline silicon layer having the predetermined orientation formed on the basis of crystallization of a substantially intrinsic amorphous silicon layer formed on the polycrystalline silicon layer of one electrically conductive type by making the polycrystalline silicon layer of one electrically conductive type as the seed crystal layer.
Moreover, in order to achieve the above-described second object, the present invention provides a crystalline silicon semiconductor device characterized in that it includes a polycrystalline silicon layer of one electrically conductive having the predetermined orientation formed on a substrate, a substantially intrinsic polycrystalline silicon layer having the predetermined orientation formed on the basis of the crystallization of a substantially intrinsic amorphous silicon layer formed on the polycrystalline silicon layer of one electrically conductive type by making the polycrystalline silicon layer of one electrically conductive type as a seed crystal layer and non single crystalline silicon layer of other one electrically conductive type formed on a polycrystalline silicon layer formed on the basis of the crystallization.
In addition, in order to achieve the above-described second object, in a method for fabricating a crystalline silicon semiconductor device forming a polycrystalline silicon layer of the predetermined thickness on a substrate, the present invention provides a method for fabricating a crystalline silicon semiconductor device characterized in that a polycrystalline silicon layer oriented on any one of a face (111), a face (110) and a face (100), a metallic catalyst layer and an amorphous silicon layer of the predetermined thickness are formed on the substrate, the amorphous silicon layer of the predetermined thickness is crystallized into a polycrystalline silicon layer having an orientation by performing heat-treatment to the amorphous silicon layer of the predetermined thickness.
As describe above, since the present invention crystallizes an amorphous silicon into a polycrystalline silicon by a metallic catalyst and heat-treatment, the present invention will have a lower cost characteristic not found in conventional methods. Specifically, an amorphous silicon can be grown in a high rate without consideration of the nature of a film, therefore, if an amorphous silicon is deposited and crystallized, a polycrystalline silicon layer of the predetermined thickness can be formed at much more rapid rate than conventional ones.
Even if the time for forming a polycrystalline silicon layer oriented in any one of the orientations (hereinafter, refer to as simply xe2x80x9corientedxe2x80x9d) and a metallic catalyst layer and the time for heat treatment are considered, it becomes a much shorter work time than that of conventional methods, therefore cost reduction can be contemplated. Moreover, since the work time is shorter, it also results in advantageously reducing loss of raw materials.
As a position of formation of a metallic catalyst layer promoting the crystallization of an amorphous silicon, it is preferable to suppose any position of inside of a polycrystalline silicon layer oriented, between a polycrystalline silicon layer oriented and an amorphous silicon layer, or on the back face of a polycrystalline silicon layer oriented contacting with an amorphous silicon layer. As a component of a metallic catalyst layer, it is preferable to select from Ni, Fe, co, Pt, Cu or Au in the viewpoint of making the crystallization of an amorphous silicon layer efficient.
As for the position relationship between a polycrystalline silicon layer oriented and an amorphous silicon layer, it will be good whether the former is positioned on the side of substrate with respect to the latter, or the former is positioned on the side of surface with respect to the latter, in either case of them, an amorphous silicon can be polycrystallized under the good orientation. Upon the upper surface and the lower surface of an amorphous silicon layer, polycrystalline silicon layers are formed, and it is practical that one of them is made p-type and the other is made n-type. Moreover, in this case, it is possible to constitute a silicon layer not on the side of a polycrystalline silicon layer oriented with a microcrystal or an amorphous silicon thin film.
As a method for forming a metallic catalyst layer within a polycrystalline silicon layer oriented, ion implantation method or plasma doping method is suitable. Moreover, it is possible to take the form in which a metallic catalyst layer is formed within a polycrystalline silicon layer through the mediation of a thin film of the metallic catalyst between a plurality of polycrystalline silicon layers. As a means for forming a metallic catalyst layer on the surface of a polycrystalline silicon layer oriented, vapor deposition method, spin coating method of metal salt solution or the like is suitable.
A metallic catalyst moves in an amorphous silicon layer during heat treatment in a direction of thickness from one side to the other side and acts to polycrystallize an amorphous silicon during this movement. Therefore, it will be enough that its amount is a trace amount, as a thickness of a thin film formed, it is common to be formed into a thickness in the order of a few angstroms.
An amorphous silicon layer is formed by vapor deposition method, p-CVD method, CVC method, sputtering method or the like. Its thickness is decided by a thickness necessary for optical absorption as a semiconductor device, in most cases, the range of 500 nm-10 xcexcm is set, however, there is also a case of a thickness of the order of 50 xcexcm.
As a temperature of heat treatment, it is preferable to be in a range of 450-700xc2x0 C., more preferably, 500-600xc2x0 C. Moreover, heat treatment may be performed in one step, or heat treatment may be performed, for example, in following two steps: a step of reducing the amount of hydrogen in a layer to less than 1%, preferably 0.3% by preheating to around 400xc2x0 C. in the atmosphere of hydrogen and a step of heating to the above-described temperature. As an atmosphere of heat treatment, hydrogen, nitrogen, Ar, halide or vacuum is preferable.
As a component of a substrate, there are various components depending on an incident direction of light to a semiconductor device. In the case where an incident light from the side of substrate is utilized, a transparent glass, a transparent ceramic, a quartz, sapphire or the like is used, and in the case where an incident light from the opposite side, SUS, Al, tungsten or a metal plate such as a metallic silicon or the like is used, It is possible that concave and convex are formed on the surface of a metal substrate and thereby increasing a short circuit current by scattering an incident light on the surface of substrate.